Many environments offer a vibration rich environment that is ideal for harvesting vibration energy. Such environments often exist in aircraft and automotive applications where the vibration experienced by an aircraft or automotive vehicle represents energy that could be used to power sensors or other remotely located devices, provided such energy can be harvested by a suitable device.
Vibration energy harvesting (VEH) can be accomplished by developing relative motion, and hence energy, between a vibrating structure and a reaction mass coupled to the structure. This mechanical energy can be converted into electrical energy by developing cyclic stress in a piezo electric structure. A simple form of this device is a cantilever beam that has piezo material attached to the surface. This is illustrated in FIG. 1. A reaction mass is attached to the tip of the beam to increase performance. When subjected to vibration, the tip of the beam tends to resist motion, thus placing the piezo material under stress. This stress results in electrical charge accumulation in the piezo material that results in an increase in voltage potential between two points of the material. However, for this topology to work efficiently, vibration energy must occur at or above the beam resonance frequency. Vibration energy with frequency content below the resonance frequency will produce very little motion between the tip mass and the base.
When the cantilever beam shown in FIG. 1 is used, the stiffness of the beam, including the piezo material, beam length (L) and the tip mass determine the lowest frequency where the VEH device will work. Additionally, piezo material is usually a ceramic and is fragile when subjected to tension loading which will limit the robustness of the VEH device and its life. FIG. 2 illustrates the relative tip displacement as a function of frequency for this device. At low frequency, the tip beam moves very little relative to the vibrating structure. Accordingly, the VEH device shown in FIG. 1, at low frequency, will provide very little power from low frequency vibrations. Vibration energy at resonance frequency will provide maximum VEH, but utility is limited by a very narrow bandwidth.
The limitation of needing to “tune” the system around the resonant frequency of the cantilever beam imposes a significant limitation in terms of efficient operation of the system shown in FIG. 1. This is because various structures often produce vibration energy over a much wider frequency bandwidth than what the system can be tuned for. The selection of the tip mass, to essentially tune the system to operate efficiently at the resonant beam frequency, means that the system will not be efficient in harvesting energy at other frequencies above and below the resonant frequency of the cantilever beam. Accordingly, a system that is not limited to efficient harvesting of vibration energy at only the resonant beam frequency, but that is able to harvest energy over a relatively wide frequency range, would be much more effective in generating electrical power from a vibrating structure.
Accordingly, there still exists a need for an apparatus able to be used with a piezo material to improve the harvesting of vibration energy at low frequencies, and also at frequencies above and below the resonant frequency of the structure from which vibration energy is being harvested. Such an apparatus would be extremely useful for powering remotely located sensors and various other components from low frequency vibration energy experienced by a piezo, beam-like structure. Such an apparatus would effectively make it possible to provide energy harvesting from a wide variety of structures experiencing low frequency vibration where such energy harvesting would have previously not been practicable.